1. Field of the Invention
The present invention relates in general to a system and method for digitally embedding information and, in particular, to a system and method for embedding digital information into digital or analog media and later detecting the embedded information using spread spectrum modulation.
2. Related Art
Embedding digital information into a digital or analog media has many important applications including counterfeiting reduction. For example, copyright information may be hidden within a digital signal (such as an image, video or audio), distributed with the digital signal and then later detected to ensure authenticity. The technique of embedding information within a work in a way that is not immediately perceptible and is hard to reproduce (also called “watermarking”) has been used for centuries in applications such as documents and currency. In current times, the widespread digital representation of images, audio, video and other signals has led to the use of a “digital watermark” for copyright protection. This marking of digital media involves hiding information under an original signal by embedding (or adding) an imperceptible signal (the mark) to the original signal.
Most marking techniques use existing spread spectrum modulation techniques to modulate information (the mark) and embed the mark within an original signal. For example, the bits composing the desired mark (such as the name of the copyright owner) are modulated using a spread spectrum sequence and added to the original signal (such as a musical work in digital form). More often, the spread spectrum sequence is added to a transformation of the original signal, which may be more sensitive to manipulation. Typically, these existing spread spectrum modulation techniques are robust to interfering noise so that the amount of energy (or distortion) that must be added to the marked signal to erase the mark is quite high. This means that it is difficult remove the mark from the original signal.
In existing spread spectrum techniques, the original signal (also known as a carrier signal) is seen as a source of noise. One problem with these existing techniques is that, as a source of noise, the original signal tends to interfere with the detection of the mark and reduce the mark detection accuracy. This is particularly a problem because the original signal is generally much stronger than any other interference and therefore is the main source of interference.
One embedding technique that uses quantization index modulation does reduce interference from the original signal. The problem, however, with this technique is that the mark is embedded in a lattice, which makes the mark highly sensitive to amplitude scaling of the signal. In other words, a slight change in the scale of the marked signal causes the mark to be erased. This makes the quantization index modulation technique practically useless in applications where a malicious attack may occur.
What is needed is a system and method for embedding information into a digital media that removes the original signal as a source of interference and obtains a large gain accurately detecting the mark within the marked digital media. In particular, the system and method would subtract the influence of the original signal from the mark thereby virtually eliminating a large portion of the interference. Elimination of this interference would greatly decrease error in the mark detection rate and greatly increase performance. In addition, the marking system should be robust and insensitive to amplitude scaling and other forms of malicious attacks. Whatever the merits of the above-mentioned systems and methods, they do not achieve the benefits of the present invention.